The present invention relates to a method for the deposition of a film of hexagonal diamond on the surface of a substrate or, more particularly, to a method for the deposition of a film of hexagonal diamond by utilizing a hydrogen plasma jet.
As is well known, it is a recently developed technology that various kinds of cutting tools and the like for working of extremely hard materials are provided with a surface coating film of diamond in an object to improve the precision of mechanical working therewith and to increase the durability of the tool edge. Such a coating film is deposited on the substrate surface as a thin layer of diamond having a crystallographically cubic structure, for example, by the plasma jet method and apparatus disclosed in Japanese Patent Kokai 1-215795 and 1-157496, according to which a plasma jet is generated by ejecting a gaseous mixture of carbon monoxide or methane as the carbon source and hydrogen and/or argon out of an anode nozzle of a special design into an arc formed by electric discharge and blown at the surface of a substrate so that a film of diamond is deposited thereon. Although the coating film of cubic diamond can be deposited in a considerably high efficiency in this known method, a problem therein is that the thus deposited diamond film consists of polycrystals of relatively coarse diamond crystallites having a diameter of 20 .mu.m or larger so that the improvement in the cutting tool obtained thereby in the precision of mechanical working with the tool cannot be as high as desired and nicking of the tool edges would readily occur to decrease the durability of the tool. An alternative method is known for the deposition of a coating film of diamond by which the size of the diamond crystallites can be much smaller, though not small enough, but this method can hardly be used in practical applications because of the unduly low velocity of deposition on the substrate surface. Japanese Patent Kokai 63-99138 discloses a plasma CVD method for forming a hard carbonaceous coating film consisting of graphite and hexagonal diamond on the surface of a machining tool, in which the substrate at a high negative potential is held apart from the plasma-generating electrodes to produce plasma of ethylene. Although some lubricity can be imparted to the surface, the thus coated tool is not always quite satisfactory in respect of the durability because the carbonaceous coating film thus formed consists mainly of graphite with a relatively minor fraction of hexagonal diamond.
The inventors accordingly have continued extensive investigations to develop an efficient method for the deposition of a coating film of diamond on the surface of a substrate without the above described problems and disadvantages in the prior art methods. In the course of their investigations, the inventors have found that the diamond film as deposited by the plasma jet method sometimes has an at least partly hexagonal crystalline structure and got an idea to seek a possibility of obtaining a quite satisfactory coating film of diamond by utilizing fully developed hexagonal phase of diamond which is a metastable phase of diamond quite different from the ordinary cubic diamond and sometimes found in meteorites. As is taught in Science, volume 155, page 995 (1967) by R. E. Hanneman, et al., Journal of Material Science, volume 22, page 3615 (1987) by T. Sekine, et al. and elsewhere, hexagonal diamond is synthetically obtained in the form of a powder by the shock-wave pressurization method, in which intense heat and extremely high pressure are applied simultaneously to a powder of graphite, but this method is absolutely not suitable for the purpose of depositing a coating film of hexagonal diamond on the surface of a substrate. No conditions of the plasma jet method are known under which the deposited coating film of diamond has a predominantly hexagonal crystalline structure and the crystallite size thereof could be small enough.